In recent years, a print speed of image forming apparatuses has been further increased. In case of an electrophotographic apparatus utilizing electrophotography among the image forming apparatuses, a circumferential velocity of a photoreceptor that is an electrostatic latent image carrier needs to be increased for a high-speed printing.
Techniques for increasing a circumferential velocity of a photoreceptor are: (a) a technique of increasing a diameter of a photoreceptor without increasing an RPM and (b) a technique of increasing an RPM without increasing a diameter of a photoreceptor. However, the technique (b) is exclusively chosen to avoid upsizing of the apparatus.
Especially in a high-speed apparatus including a photoreceptor of a high circumferential velocity, an essential technique is a technique of stably charging the photoreceptor in a state where a surface potential of the photoreceptor is at a predetermined potential (set charging potential). This is because an unstable surface potential of the photoreceptor results in (i) the phenomenon called “base fogging” caused by changes of the surface potential and (ii) decrease in print density. Further, a too high surface potential contributes to “deterioration of a photoreceptor” in consideration of a withstand voltage for a charging potential of the photoreceptor.
Charging devices which charge the surface of a photoreceptor are classified into (i) contact-type charging devices using a charging roller, a charging brush, or the like and (ii) noncontact-type charging devices typified by a corona charging device using corona discharge.
In the contact-type charging devices, electrons are attached onto a photoreceptor while a charging member such as roller or brush is brought into direct contact with the photoreceptor. This realizes an efficient charging and circumvents the need for a high voltage as a voltage applied to the charging member. Besides, the contact-type charging devices generates an extremely small amount of ozone, which is a cause of environmental pollution, and are excellent in terms of ecological activities.
However, in the contact-type charging devices, a voltage can be usually applied only to a 3 to 5 mm wide nip region where the photoreceptor and a contact member come into contact with each other. Because of this, the contact-type charging devices have a small voltage application area. For the reason of this drawback, it is possible to charge the photoreceptor at a predetermined potential in a low-speed apparatus, but it is difficult to charge the photoreceptor at the predetermined potential in a high-speed apparatus. In addition, since the charging member is in direct contact with the photoreceptor, dust or the like on the contact member is likely to adhere to the photoreceptor. In the high-speed apparatus adopting the contact-type charging device, damage to the surface of the photoreceptor caused by the adherents is a big problem.
On the other hand, since the noncontact-type. charging devices use corona discharge, the noncontact-type charging devices requires a higher voltage than that of the contact-type charging device and therefore generates ozone. However, the noncontact-type charging devices have a merit that a large voltage application area can be secured, and makes it possible to charge the surface of the photoreceptor at the predetermined potential even in a high-speed apparatus. Moreover, since the noncontact-type charging device eliminates a direct contact with the photoreceptor, damage to the surface of the photoreceptor occurs less often.
Now, referring to FIG. 45, the structure of a corona charging device is briefly described. As illustrated in FIG. 45, the corona charging device has charger lines 141 which are subjected to application of a high voltage. The charger lines 141 are held and shielded in a charger case 142. The charger case 142 has an open surface facing a photoreceptor. The charger case 142 is located in such a manner that the charger lines 141 face the photoreceptor via the open surface. The charger lines 141 are located in such a manner that an axial direction of the charger lines 141 is orthogonal to a rotational direction of the photoreceptor.
In such a structure, the voltage application area is determined depending upon a width of the charger case 142 (width of the photoreceptor in the rotational direction of the photoreceptor). The width of the charger case 142 can be increased with increase of the number of charger lines 141 provided in the charger case 142.
The corona charging devices are classified into corotron charging devices and scorotron charging devices as described in Japanese Unexamined Patent Publication No. 142904/1998 (Tokukaihei 10-142904; published on May 29, 1998), for example.
The corotron charging devices and the scorotron charging devices are different in the presence or absence of a grid electrode. The corona charging device illustrated in FIG. 45 is a scorotron charging device wherein a meshed grid electrode 143 which is subjected to a bias voltage is disposed between the charger lines 141 and the photoreceptor.
The charger lines 141 vibrate when a high voltage is applied thereto. In the corotron charging device which does not include the grid electrode 143 has the problem that a distance between the photoreceptor and the charger liens 141 changes during voltage application and a charging potential varies in a direction orthogonal to the rotational direction of the photoreceptor.
On the contrary, in the scorotron charging device including the grid electrode 143, even when the charger lines 141 vibrate, a current supplied from the charger lines 141 and passing through the grid electrode 143 is absorbed at a bias voltage applied to the photoreceptor and the grid electrode 143, which regulates the charging potential. This saturates and uniforms the surface potential of the photoreceptor.
A saturation value that is a charging potential of the photoreceptor can be controlled by a voltage applied to the grid electrode 143. Assume that a discharge potential which occurs when a high voltage is applied to the charger lines 141 is represented by “A”, and a bias voltage which is applied to the grid electrode 143 is represented by “B”. When A>B, the saturation value takes B. When A≦B, the saturation value is below B.
Thus, as compared with the corotron charging device, the scorotron charging device is of more complex structure and inferior in charging efficiency due to provision of the grid electrode 143. However, the scorotron charging device has been used in most cases as a charging device using a corona discharge because the scorotron charging device has an advantage of controlling a charging potential and is capable of uniformly charging the surface of the photoreceptor.
The grid electrode 143 is realized by an electrically conductive thin plate made of a material such as SUS (0.1 mm thick) having a plurality of slits or polygonal openings, such as hexagonal openings, formed by etching or the like method.
Conventionally, in case of the grid electrode 143 having slits as the openings, a grid electrode having a slit width ranging from 1.0 mm to 1.4 mm and a slit pitch ranging from 1.16 mm to 1.56 mm is used in most cases. In case of the grid electrode 143 having hexagonal openings as the openings, a grid electrode having a 2.5 mm to 3.4 mm diameter of a circumcircle around the hexagonal opening and a 2.75 mm to 3.65 mm pitch between the circumcircles is used in most cases.
However, the conventional corona charging device has the following problems to be solved.
A circumferential velocity of the photoreceptor has been increased to meet a demand for high-speed printing. However, it is difficult to make a potential of the photoreceptor reach the predetermined potential only by the measure of increasing a voltage application area by increasing the width of the charger case.
In other words, in the corona charging device, the voltage application area can be increased with increase of the width of the charger case 142 (width in a rotational direction of the photoreceptor). However, various kinds of known members used in the Carlson process, including not only the charging device, but also a developing device, cleaning device, and a transfer device, are disposed around the photoreceptor. As such, increase of the voltage application area by increasing a width of the charger case 142 has a ceiling. Excessive increase of a width of the charger case 142 takes up a space for disposing other members. This results in increase of a drum diameter of the photoreceptor, thus upsizing the image forming apparatus.
Further, Japanese Unexamined Patent Publication No. 137368/2000 (Tokukai 2000-137368; published on May 16, 2000) discloses the arrangement in which a plurality of scorotron charging sections are provided side by side around a photoreceptor drum, wherein the charging section located. on the most upstream side in the rotational direction of the photoreceptor drum includes openings of the highest aperture ratio and the charging section on the most downstream side in the rotational direction of the photoreceptor drum includes openings of the lowest aperture ratio.
However, even the arrangement disclosed in Japanese Unexamined Patent Publication No. 137368/2000, i.e. the arrangement in which a plurality of corona charging devices are provided side by side around the photoreceptor drum cannot solve the problem of up sizing.
Apart from the measure of increasing the voltage application area, application of an extremely high voltage to the charger lines 141 is also considered as a measure for causing a potential of the photoreceptor to reach a predetermined potential in the high-speed apparatus. However, such a measure is not preferable because it inevitably increases the amount of ozone generation and it runs counter to ecological activities.
It was found out that further increase of a circumferential velocity of the photoreceptor makes it difficult to uniformly and stably charge the photoreceptor at a predetermined potential, which is determined by the grid electrode.
FIG. 46 illustrates a result of the examination on a relation between a current flown to the charger lines (hereinafter also referred to as “wire current”) and a surface potential of the photoreceptor (hereinafter also referred to as “drum surface potential”) in an image forming apparatus serving as an electrophotographic apparatus and including the scorotron charging device in a state where a circumferential velocity of the photoreceptor is increased stepwise to a higher value than a conventional value. Here, assume that the upper limit of the wire current is 900 μA. This is because the amount of ozone generation remarkably increases when the wire current exceeds 900 μA.
In FIG. 46, a horizontal axis represents the wire current, and vertical axes represent the drum surface potential and a voltage applied to the charger lines 141 (hereinafter also referred to as “wire voltage”). Assume that a voltage applied to the grid electrode 143 (grid potential) was 650 (—V) that is a target potential for the drum surface potential. The grid electrode 143 was a 0.1 mm-thick thin plate made of SUS including a plurality of slits, wherein a width of an electrode line provided between the slits was in the range from 0.15 mm to 0.17 mm (error of 0.16 mm±0.01 mm), and a slit width was 1.2 mm (slit pitch ranging from 1.35 mm to 1.37 mm (error of 1.36 mm±0.01 mm)). The two charger lines 141, which were made of Φ60-μ tungsten wire, were placed side by side in a rotational direction of the photoreceptor. The charger lines 141 shared one electrode.
As illustrated in FIG. 46, when the circumferential velocity is 240 mm/sec, the drum surface potential reaches −650 V, which is a grid potential, and saturates. However, when the circumferential velocity is 420 mm/sec, the drum surface potential stops rising before reaching the grid potential. When the circumferential velocity is further increased to 600 mm/sec, the drum surface potential becomes far below the grid potential.
As a result, it was found out that the relation between the wire current and the drum surface potential was significantly influenced by the circumferential velocity of the photoreceptor, and that the drum surface potential became far below the grid potential as the circumferential velocity increased. It was found out that curves representing the drum surface potential relative to the wire current were similar to each other regardless of the circumferential velocity, and that the curves shifted to 0V as the circumferential velocity increased.